1. Field of the Invention
The present invention relates to an apparatus for transferring substrates such as semiconductor wafers, and also a method of transferring substrates.
2. Description of the Related Art
A process of manufacturing semiconductor devices includes a step of performing heat treatment on a substrate such as a semiconductor wafer. Recently, a heat treatment apparatus designed to process a large number of wafers in a batch is employed to perform the heat treatment. The apparatus has a processing vessel in which wafers will be heat-treated. To be heat-treated, the wafers are transferred from a wafer carrier made of resin (e.g., Teflon.TM.) onto a wafer boat made of quartz or fused silica. The wafer boat containing the wafers is loaded into the processing vessel. In the processing vessel, the wafers are heat-treated. After the heat treatment, the wafer boat containing the wafers is unloaded from the reaction chamber, and the heat-treated wafers are transferred back into the carrier.
The wafers are transferred from the wafer carrier onto the wafer boat, and vice versa, by means of a transfer apparatus located between the wafer wafer carrier and the wafer boat. The transfer apparatus has a base unit and forks, for supporting a wafer. The base unit is moved up and down and rotated in the horizontal plane, thereby being set at a position where a transfer operation can be performed. The forks are moved back and forth in the horizontal plane with respect to the base unit. The base unit and the forks are moved in accordance with patterns previously set in a control mechanism. The wafers are transferred in accordance with above movements of the base unit and the forks.
In the actual transfer operation, the forks must take appropriate positions with respect to the wafer carrier, the wafer boat, and the wafers.
However, the wafer carrier or the wafer boat does not always take the same position or assume the same attitude. Nor do the wafer-supporting grooves of the carrier or the boat remain in the same condition at all times. Actually, the carrier and the boat may be inclined, displaced, or deformed. The boat may be deformed due to the heat it received during the heat treatment, or may be deformed while being washed to remove contaminant adhered to it during the heat treatment. Further, the wafer boat having inherent displacement may be replaced by another one. Still further, the wafer carrier is likely to be deformed either partly or entirely since it is made of resin.
If the wafer carrier or the wafer boat is displaced, or if the wafers are inappropriately arranged in the carrier or boat, the forks fail to transfer the wafer between the carrier and the boat in a desired manner since the operating positions of the forks of the transfer apparatus controlled in accordance with basic operating patterns are in improper positions with respect to the wafers. More precisely, the forks can neither support the wafer in a desirable position or attitude, nor precisely insert the wafer into the groove of the target wafer-support (the boat or the carrier) or into any wafer-support groove at all. Consequently, the wafer may fall from the forks and be broken, or may abut on the next one, inevitably damaging the same. In the worst possible case, the forks may hit and topple the wafer boat.
Therefore, when a new wafer support, i.e., a wafer boat or a wafer carrier, is installed in place of the one hitherto used, the operating pattern of the transfer apparatus must be adjusted, (the adjustment is called teaching), so that it may transfer wafers appropriately from and onto the new wafer support. Conventionally, the teaching is performed by a skilled operator.
Hitherto, the teaching is performed as follows. The operator enters the heat treatment apparatus incorporating the transfer apparatus, and minutely moves the body of the apparatus and the forks, on a trial-and-error basis, while visually confirming the vertical movement, swinging, and horizontal movement of each fork, so that each form may assume a position sufficiently appropriate with respect to the wafer support at any time during the wafer-transferring operation.
The results of the teaching largely depends on the skill and experience of the operator, and are not reproducible in most cases. Further, the wafer support is not always identical in size to others of the same type. Still further, the wafer-supporting grooves of the wafer support are usually displaced from their desirable positions. Inevitably, the teaching requires much labor and much time on the part of the operator, and can hardly achieve reliable adjustment of the operation pattern of the transfer apparatus.
Most heat treatment apparatuses for performing heat treatment on a batch of semiconductor wafers are small and have many components arranged in a limited space. The operator cannot have an easy access into them. Inevitably it would be difficult for the operator to perform the teaching of the transfer apparatus which is incorporated in the heat treatment apparatus. When the heat treatment apparatus is accessed by the operator, its interior may likely be contaminated.
The basic operating pattern of the transfer apparatus may have some displacement from the proper operation. In this case, even if the actual displacement is small, the transfer apparatus fails to operate in the predetermined pattern if other undesirable situations take place in the heat treatment apparatus or if the forks or the wafer support hold wafers in an improper manner. As a consequence, the wafers or the forks may be damaged in some cases. It remains unknown until the wafer or the forks are damaged that an inappropriate operating pattern has been set for the transfer apparatus.